The present invention relates to improved metal tribological surfaces, and to lapping methods and systems for producing such surfaces.
In order to reduce friction and wear in mechanically interacting surfaces, a lubricant is introduced to the zone of interaction. As depicted schematically in FIG. 1A, under ideal lubricating conditions, the lubricant film 20 between opposing surfaces 32 and 34, moving at a relative velocity V, forms an intact layer which permits the moving surfaces to interact with the lubricant. Under such conditions, no contact between surfaces 32 and 34 occurs at all, and the lubricant layer is said to carry a load P that exists between the opposing surfaces. If the supply of lubricant is insufficient, a redaction in the effectively of the lubrication ensues, which allows surface-to-surface interactions to occur.
As shown schematically in FIG. 1B, below a certain level of lubricant supply, the distance between opposing, relatively moving surfaces 32 and 34 diminishes because of load P, such that surface asperities, i.e., peaks of surface material protruding from the surfaces, may interact. Thus, for example, an asperity 36 of surface 34 can physically contact and interact with an asperity 38 of surface 32. In an extreme condition, the asperities of surfaces 32 and 34 carry all of the load existing between the interacting surfaces. In this condition, often referred to as boundary lubrication, the lubricant is ineffective and the friction and wear are high.
Grinding and lapping are conventional methods of improving surface roughness and for producing working surfaces for, inter alia, various tribological applications. FIGS. 2A and 2B schematically illustrate a working surface being conditioned in a conventional lapping process. In FIG. 2A, a working surface 32 of a workpiece 31 faces a contact surface 35 of lapping tool 34. An abrasive paste containing abrasive particles, of which is illustrated a typical abrasive particle 36, is disposed between working surface 32 and contact surface 35. Contact surface 35 of lapping tool 34 is made of a material having a lower hardness with respect to working surface 32. The composition and size distribution of the abrasive particles are selected so as to readily wear down working surface 32 according to plan, such as reducing surface roughness so as to achieve a pre-determined finish.
A load is exerted in a substantially normal direction to surfaces 32 and 35, causing abrasive particle 36 to penetrate working surface 32 and contact surface 35, and resulting in a pressure P being exerted on a section of abrasive particle 36 that is embedded in working surface 32. The penetration depth of abrasive particle 36 into working surface 32 is designated by ha1; the penetration depth of abrasive particle 36 into contact surface 35 is designated by hb1. Generally, abrasive particle 36 penetrates into lapping tool 34 to a greater extent than the penetration into workpiece 31, such that hb1>>ha1.
In FIG. 2B, workpiece 31 and lapping tool 34 are made to move in a relative velocity V. The pressure P, and relative velocity V of workpiece 31 and lapping tool 34, are of a magnitude such that abrasive particle 36, acting like a knife, gouges out a chip of surface material from workpiece 31.
At low relative velocities, abrasive particle 36 is substantially stationary. Typically, however, and as shown in FIG. 2B, relative velocity V is selected such that a corresponding shear force Q is large. Because the material of lapping tool 34 that is in contact with abrasive particle 36 is substantially unyielding (i.e., of low elasticity) with respect to the particles in the abrasive paste, these particles are usually ground up quite quickly, such that the abrasive paste must be replenished frequently.
In the known art, grinding, lapping, polishing and cutting are carried out on materials such as metals, ceramics, glass, plastic, wood and the like, using bonded abrasives such as grinding wheels, coated abrasives, loose abrasives and abrasive cutting tools. Abrasive particles, the cutting tools of the abrasive process, are naturally occurring or synthetic materials which are generally much harder than the materials which they cut. The most commonly used abrasives in bonded, coated and loose abrasive applications are garnet, alpha alumina, silicon carbide, boron carbide, cubic boron nitride, and diamond. The relative hardness of the materials can be seen from Table 1:
TABLE 1Knoop HardnessMaterialNumbergarnet1360alpha-alumina2100silicon carbide2480boron carbide2750cubic boron nitride4500diamond (monocrystalline)7000The choice of abrasive is normally dictated by economics, finish desired, and the material being abraded. The above-provided list of abrasive materials is in order of increasing hardness, but is also, coincidentally, in order of increasing cost, with garnet being the least expensive abrasive material and diamond the most expensive.
Generally, a soft abrasive is selected to abrade a soft material and a hard abrasive to abrade harder types of materials in view of the cost of the various abrasive materials. There are, of course, exceptions such as very gummy materials where the harder materials actually cut more efficiently. Furthermore, the harder the abrasive grain, the more material it will remove per unit volume or weight of abrasive. Super-abrasive materials include diamond and cubic boron nitride, both of which are used in a wide variety of applications.
Conventional lapping methods and systems generally have several distinct deficiencies, including:                The contact surface of the lapping tool is eventually consumed by the abrasive material, requiring replacement. In some typical applications, the contact surface of the lapping tool is replaced after approximately 50 workpieces have been processed.        The lapping processing must generally be performed in several discrete lapping stages, each stage using an abrasive paste having different physical properties.        Sensitivity to the properties of the abrasive paste, including paste formulation, hardness of the abrasive particles, and particle size distribution (PSD) of the abrasive particles.        Sensitivity to various processing parameters in the lapping process.        
Various improvements to these conventional lapping methods and systems have been disclosed in U.S. Pat. No. 7,134,939 to Shamshidov et al., and in U.S. Patent Publication No. 2007/0123152 to Shteinvas et al., both of which applications are assigned to Fricso, Ltd.
These advancements notwithstanding, there is a recognized need for, and it would be highly advantageous to have workpieces and tribological systems having metal working surfaces that exhibit improved tribological properties. It would be of further advantage to have a lapping method and system that overcome various deficiencies of the known lapping technologies, and that produce such improved metal working surfaces.